Photoluminescence or fluorescence is the light emission caused by light excitation. Electroluminescence occurs by the emission of light from a phosphor in response to a sufficiently high electric field developed across the phosphor. Phosphor refers to those materials that emit light in response to the application of a field across the material. Thin film electroluminescent devices have a basic structure comprising a phosphor film or layer sandwiched between two electrodes. Scintillation or radioluminescence is the emission of light excited by high energy particles, beam or radiation rays like X-ray, Gamma- or Beta rays. Cathodoluminescence is a luminescence phenomenon excited by electron beams. The invented phosphors can be used for all these kinds of luminescence in applications for solid state lighting, display, detection and sensing.
There is strong commercial interest in achieving a wide spectral range in electroluminescent phosphors for visible display application and in particular for making color flat panel displays. Mn2+ doped zinc sulfide (ZnS:Mn) phosphor is perhaps the most studied phosphor due to its potential applications in solid state lighting, displays and sensing technologies. ZnS is a direct transition semiconductor with the largest band gap among II-VI compounds, and is an excellent host for doped phosphors. In ZnS:Mn, Mn is isovalent, having the same valence as the host cation, and therefore, Mn2+ ions can substitute the Zn2+ sites without producing any obvious defects or vacancies. The chemical bonding characteristics of Mn2+ and Zn2+ are expected to be similar since they have a common ionization state and, hence valence. Thus, a Mn2+ luminescence center in ZnS:Mn is electronically neutral in the lattice in equilibrium, and this makes ZnS:Mn an ideal phosphor with intense photoluminescence, electroluminescence, upconversion luminescence and triboluminescence.
In addition to the excellent luminescence properties, ZnS:Mn is very stable and easy to synthesize. The Mn2+ ionization energy has been reported to be 3.0 eV to 3.5 eV below the valence band maximum. Since the ionization energy of Mn2+ is large, it is not usually ionized under normal operation in electroluminescence devices. These characteristics make ZnS:Mn an ideal phosphor for practical applications. Therefore, ZnS:Mn is one of the most widely investigated doped phosphors, not only in bulk but also as nanoparticles.
ZnS:Mn has two shortcomings in terms of practical applications. As the emission is from the d-d transition of Mn2+, which is forbidden, the resulting luminescence is not very efficient and the luminescence lifetime is long (millisecond range). Efforts have been made to solve these two challenges by preparing nanosized particles of ZnS:Mn. It was predicted that the quantum size effect may shorten the luminescence lifetime and increase the luminescence efficiency. Indeed, it has been reported that the luminescence lifetime of Mn2+ in ZnS:Mn nanoparticles changed from milliseconds in the bulk to nanoseconds in nanoparticles. However, it was later determined that the shortening is most likely due to the lifetime components from defects or surface states. Many methods have been successfully applied to enhance the luminescence of ZnS:Mn nanoparticles, such as surface passivation with polymers or silica, encapsulation in zeolites, and surface plasmon coupling. Therefore, it would be desirable to enhance the luminescence intensity of Mn2+ in ZnS phosphors by simply co-doping with Eu2+ ions, and explore their applications as a new phosphor and investigate the mechanisms responsible for the enhancement.
As a sensitive method, fluorescence detection has been extensively studied and used since early years of last century. Among those detection methods, fluorescence intensity ratio (FIR) is an advanced technology based on the relative intensity changes of two emissions rather than the only change in one emission peak. This advantage makes it independent of the local concentration of fluorescence probe. Thus, the FIR is very reliable and has found its applications in a variety of sensing and detection fields, such as temperature measurement, biological sensing, and trace water detection. However, it has been rarely applied for radiation detection. Currently, most radiation detections are based on the detection of light or electric signals induced by radiation. A ratio monitoring of two radiation-dependent emission intensity changes would be a sensitive and reliable method for radiation detection. Changes in the emissions of Eu2+- and Mn2+-doped CaF2 and MgF2 phosphors can be used for radiation detection Zinc sulfide (ZnS) is a simple and widely used host material that has excellent capability to be doped with multiple metal ions. Doped ZnS phosphors are able to emit light in a wide range of wavelengths.